The present invention relates to the general field of reducing jet noise at the outlet from a turbomachine nozzle. It relates more particularly to a cowl for a turbomachine nozzle of the separate-stream type that is provided with patterns for reducing jet noise.
Nowadays, sound pollution has become a major concern for engine manufacturers who are being confronted more and more with the acoustic nuisance of their turbomachines. The sources of noise in a turbomachine are numerous, but it has been observed that the jet noise at the outlet from the nozzle is the noise that predominates during airplane takeoff. Since certification authorities are becoming more and more demanding in terms of sound emission from turbomachines, engine manufacturers are being required to reduce the noise from their turbomachines, and in particular to reduce jet noise at the outlet from nozzles.
Typically, a separate-stream nozzle for a turbomachine has a primary cowl centered on the longitudinal axis of the turbomachine, a secondary cowl arranged concentrically around the primary cowl so as to define a first annular channel in which an outer stream (or cold stream) flows, and a central body arranged concentrically inside the primary cowl so as to define a second annular channel in which an inner stream (or hot stream) flows, with the primary cowl extending beyond the secondary cowl.
In such a nozzle, the jet noise comes from mixing taking place both between the hot and cold streams and between the cold stream and the outer air surrounding the nozzle. This noise is noise occupying a broad frequency band and it is generated by two types of sound source: high frequency noise coming from small turbulent structures of the mixing between the streams, which noise is perceived essentially while close to the nozzle; and low frequency noise coming from large vortex structures that appear a long way from the jet.
In order to reduce jet noise, one of the means used is to increase the efficiency with which the streams mix together. For this purpose, it is known to provide at least one of the cowls of the nozzle with a plurality of repetitive patterns that are distributed around the entire circumference of the trailing edge of the cowl. By putting such patterns into place on the trailing edge of the cowl of the nozzle, mixing between the streams is achieved by creating turbulence (or vortices) close to the nozzle in order to dissipate kinetic energy better, and consequently reduce the turbulent intensity of the large vortices that constitute the major sources of noise.
By way of example, U.S. Pat. No. 6,532,729 provides for fitting the trailing edges of the primary and secondary cowls of the nozzle with a plurality of repetitive patterns of triangular shape (referred to as chevrons) that serve to enhance mixing between the streams flowing past either side of these patterns.
Such patterns serve to enhance mixing between the streams, in particular by reducing the low frequency component of the jet noise. This reduction, which needs to be increased, is nevertheless obtained at the cost of a penalty on the performance of the turbomachine, since the efficiency of the nozzle is degraded.